It is not known whether transcriptional activation of individual IAP elements is random or if particular elements are activated in different cells because of availability of factors that favor expression of those elements. We have compared sequences of active IAP elements from normal and transformed B-cells to address this question. Mouse plasmacytomas generally express high levels of IAP RNA. We have characterized the IAP genes expressed in a myeloma to determine whether the same IAP genes that are expressed in B-cells are expressed at higher levels, or whether new IAP genes are activated. Previous work has established that a limited and highly characteristic set of IAP elements (designated LS) is expressed in normal mouse B-cells (Kuff AR 91). None of 50 IAP cDNAs isolated from the MPC11 myeloma was of the B-cell type by hybridization to LS IAP specific probes. Twenty-two MPC11 cDNA LTRs were sequenced and found to represent two closely related IAP classes. Their sequences differed markedly from those of the LS IAP class. The IAP cDNAs expressed in this myeloma are very similar to one another in the U3 region of the LTR, where sequences that control transcription are located. These results suggest that expression of IAP elements in transformed cells is not due to random activation, but may be determined by cell specific factors that interact with particular sequence variations within the IAP control regions.